03 May 2012: Report

Will Solar Windows Transform Buildings to Energy Producers?

The vast amount of glass in skyscrapers and office buildings represents enormous potential for an emerging technology that turns windows into solar panels. But major questions remain as to whether solar windows can be sufficiently inexpensive and efficient to be widely adopted.

by dave levitan

If you picture the glittering glass skyscrapers that dot America’s cities, it becomes clear why the idea of using that vast window space to generate solar power is gaining traction. In 2009 alone, 437 million square feet of windows were installed in non-residential buildings in the United States. That many square feet of standard solar panels would generate around 4 gigawatts of power, roughly the total installed solar capacity in the U.S. today.

Such potential is leading engineers and entrepreneurs to more intensively explore the idea of turning windows into solar-power producers. Solar windows, a subset of the growing field known as building-integrated photovoltaics, are based on the concept that a window doesn’t need to be 100 percent transparent, and a solar panel doesn’t need to be 100 percent opaque. Several ways currently exist to turn a window into a power-generating device, from thin-film silicon, to dye-sensitized solar cells, to tiny organic cells.

Some experts think the field is poised to take off, and although the world may not see an all-solar skyscraper for a while, a number of companies are

‘The challenge is whether you can get the cost down and the electricity generation up,’ says one expert.

promising commercial-scale production of various solar windows in the next two years. Still, the cost and technical hurdles facing this fledgling technology could get in the way of a future filled with towering, emission-free power plants. Like other cutting edge alternative energy sources, energy-generating windows could become a mainstay of a greener future in the coming decades, or they could prove to be impractical and produce only a fraction of solar-powered electricity.

“The challenge is whether you can get the cost down and the electricity generation up,” says Sarah Kurtz, a scientist with the U.S. government’s National Renewable Energy Laboratory (NREL) in Colorado. “There are lots of different schemes and strategies, and creativity will be the name of the game. If you can get the cost to the place where those windows don’t really cost any more than conventional windows, it obviously makes sense to go ahead and have your windows generate electricity.”

Building-integrated photovoltaics, or BIPV, is moving slowly, with solar panels now doubling as walls, shingles, and other parts of buildings. MJ Shiao, a senior analyst at GTM Research, a market analysis group in Cambridge, Massachusetts, says the market still represents only around 1 percent (a few hundred megawatts last year) of solar power being installed around the world, and that’s mostly rooftops or semi-opaque skylights. Windows pose a greater challenge than rooftops or walls because of the need to actually see through them. So far, very few examples of skyscrapers with solar windows exist; the highest profile site is the Willis Tower (formerly Sears Tower) in Chicago, where Pythagoras Solar installed a small prototype in 2011.

Several technologies have emerged for solar windows, though none have yet taken off in a meaningful way. But one company that says it is close to commercial deployment is New Energy Technologies, based in Columbia,

The rate at which a solar panel turns the sun’s energy into electricity is a special concern for solar windows.

Maryland. It has developed a method for spraying tiny organic solar cells onto windows in a see-through coating that lets in 40 to 80 percent of sunlight, absorbing the rest. With 10 patent filings pending and no commercial prototypes yet in the field, the company is divulging few details. But the spray-on method could reduce production costs dramatically. Recently, the company announced the development of a large solar cell — 170 square centimeters — made in collaboration with NREL, which could make adding the cells to windows even cheaper.

Despite the company’s progress, its technologies highlight one of the major obstacles to solar windows: efficiency. The rate at which a solar panel turns the sun’s energy into electricity is a concern for all types of solar power, but especially for windows. “The challenge is that the light you see, if you absorb that and use it to make electricity, that means you don’t have a window anymore,” says Kurtz.

To date, the record efficiency for an organic solar cell is 10 percent, and production line efficiencies never get up to the record levels. While traditional solar panels are now producing power with 15 to 20 percent efficiency, efficiency levels for solar windows of roughly 5 percent are unlikely to be economical.

“Look at it from a physicist’s point of view,” Kurtz says. “A solar panel that’s put out in the desert in a nice location with lots of sunshine may have something on the order of a one-year payback. If that [panel] sits out there for another 20 years, you get that much return on your investment for society.” If a solar window can only achieve one-third the efficiency of a solar panel, then it will take three times as long to pay back the investment.

But some experts think it’s just a matter of time before efficiencies rise high enough — and costs drop low enough — to make solar windows a sound

Some experts think it’s just a matter of time before solar windows are a sound investment.

investment. Andreas Athienitis, a professor of mechanical engineering at Concordia University in Montreal who is working on technologies for solar windows, says more mature technologies like thin-film silicon might represent a short- and mid-term solution for BIPV, until organic cells can catch up and meet long-term goals. “I think eventually it will be a big market,” he says, but the adoption is slow because “it’s a disruptive technology.”

Another company using organic solar cells, Heliatek, based in Germany, has panels that can achieve 8 percent efficiency. The company’s organic cells use molecules called oligomers rather than traditionally used polymers — basically, short rather than long collections of atoms — which means cheaper, more precise application of the cells. Heliatek says it expects that within five years it can manufacture solar cell windows in the 50 cents-per-watt range, making them competitive with other solar technology.

Spain-based Onyx Solar offers a number of solar glass technologies. However, its windows only allow up to 30 percent of sunlight through, so a lot of light inside the building is lost. In varying formations, though, Onyx says its amorphous-silicon solar glass — a type of thin-film silicon cell — can get up to 9-percent efficiency.

But such efficiencies don’t take into account some of the practical limitations of actually covering a skyscraper with solar windows.

“The optimal installation for solar is you want it to be facing south, you want a slight tilt to it, and you want good solar access, so you don’t want anything to shade those panels,” says Shiao, of GTM Research. “The problem with skyscrapers is suddenly you’re putting them in vertical orientation, there’s

The economics of solar windows may be most compelling for new construction of net-zero energy buildings.

only one south side to the building, and chances are that skyscraper is next to another skyscraper, which is going to shade that side of the building.”

Such challenges have left Shiao and other experts skeptical that solar windows will have a bright future. “There are a lot of technical and design challenges, which quite honestly aren’t going to be fixed,” Shiao says. “It doesn’t make sense almost at any cost, unless you’re getting the panels for free or something, to really install that system on those big structures.”

Oxford’s technology involves a type of cell for solar windows called a dye-sensitized solar cell. Dye-sensitized cells use a photo-electrochemical process to generate power and can be made relatively cheaply. Oxford’s transparent panels are so far getting around 6 percent efficiency, and the company hopes to bring them to market late next year.

Nazir Kherani, a professor of engineering at the University of Toronto, believes that the economics of solar windows may be most compelling for new construction with a focus on net-zero energy buildings — not for retrofitting existing skyscrapers. “With sufficient attention to design and seamless engineering, it is conceivable that we may see such buildings gradually evolving into net-zero communities, villages, and towns,” Kherani says.

Daunted by high up-front costs, U.S. homeowners continue to shy away from residential solar power systems, even as utility-scale solar projects are taking off. But with do-it-yourself kits and other installation approaches now on the market, Dave Levitan writes that residential solar is having modest growth.READ MORE

Several companies involved in solar window production say they are within a year or two of scaling up or bringing a product to market, and they maintain that cell efficiencies will continue to rise and prices continue to fall, as is the case with solar panels.

What continues to drive the inventors and entrepreneurs involved in developing solar windows is the enormous potential for energy savings. Buildings accounted for 41 percent of all electricity consumption in the U.S. in 2010, more than transportation or industry. Taking a bite out of that with power-generating windows is an alluring target.

“I wouldn’t write off the possibility,” Kurtz says. “How soon will it happen? I find it’s really dangerous to predict the future.”

COMMENTS

Over a 30-year period, initial building costs (design + construction) account for about 2 percent of the total operations and maintenance costs represent 6 percent. Personnel costs will equal the remaining 92 percent.

While design and construction costs account for 2 percent only, the building envelope accounts —accordingly — for a minimal part over these 30 years.

In line, all international councils for green building and the industry state that the greatest improvement that you can undertake in order to enhance the overall efficiency of the building, is to create high performing facades.

BIPV is a clear example of technology contributing to this high performing facades, not only generating onsite, but also being deployed as IG units, providing thermal and acoustic insulation, UV/IR filtering effect, daylighting and other passive bioclimatic properties.

Talking about daylighting, I have seen some building designs which are hard to understand. 100 percent transparent glazed facades that provide 90-100 percent light transmission. All these buildings, after a short period of time, need to incorporate a shading device to protect common areas such as entrances, which are overexposed to sunlight, by using fabric elements, interior blinds and other materials. Sometimes the designer foresees this issue and looks for a frit patterned glass for those buildings with high sun exposure.

Alternatively, I checked out that Spanish company, Onyx Solar, (www.onyxsolar.com) and easily understood the functionality of the PV Skylight that they carried out at San Anton Market in Madrid http://www.onyxsolar.com/photovoltaic-skylight.html .

I read it provides 20 percent light transmission and totally transparent views from the interior to the exterior, and that is counting on thin film technology. Incoming visible light is more than enough to naturally illuminate the building (I visited the place), at the same time that protected against overheating. People are confused about the trade-off between efficiency and transparency. Everything is about balance 50 percent light transmission glass is also available — even 60 or 70 percent — for thin film technologies, but some understanding need to be clarified in further detail.

On the other hand, people forget about the aesthetic, fashionable design that current BIPV offer the market. This technology is ready for large scale utilization I believe just more innovative thinking is needed within the industry as well as real commitment towards sustainability.

I rather prefer cities and architectural design to be reduced to brick, cubic, impersonal buildings, and to place some old-fashioned PV panels on top of my roof maybe we all can bring our art and heritage payback time down to a few months.

Posted by
Carlos B. Vazquez
on 03 May 2012

"If you can get the cost to the place where those windows don’t really cost any more than conventional windows, it obviously makes sense to go ahead and have your windows generate electricity.”

Wouldn't you just have to get the cost to a place where the windows don't cost more than conventional windows plus the projected energy costs? The pv windows would be generating electricity, eliminating the need to pay (at least in part) for electricity.

Also, a great building with slanted windows and and a southerly exposure is also in Chicago.
The Smurfit-Stone Building doesn't have any obstructions and would be recognizable to for marketing purposes. Just a suggestion, if any pv window folks are reading this.

Posted by
Patrick CoatarPeter
on 17 May 2012

I like your article, but I disagree in the focus that the industry should have.

The solar industry focuses in efficiency and payback, and they should. You have to build from cero a new facility and invest money that you expect to recoup. But, and this is a big but, the BIPV industry should NOT focus in efficiency and payback. They should focus in real integration and price, so they can replace parts of the building with new materials that produce energy. In this way, you can build with this new material, instead of the traditional ones, at the same cost and additionally you will produce energy. Think about it if you want that building you have to expend the money anyway, if you can choose between a window that produce electricity and one that doesn’t (if they are virtually indistinguishable in price and appearance) then the efficiency and payback do not make much sense because from the beginning the upgrade cost cero.

The problem is that the industry focuses too much in this two concepts and that is why the R&D efforts are miss directed. Later on, when everybody is building with this new materials and the mindset change from buildings that consume energy, to buildings that produce energy, we can further improve the technologies and bust efficiency.

Posted by
Adalberto Gallardo
on 31 May 2012

Big expectations for future applications with this French startup Wysips which patented the world's first transparent photovoltaic film, flexible and free from size constraints... A solution to product energy with building glasses (see their website : http://www.wysips.com/) I'm just proud that in France we still have ideas!

Posted by
MLeitzelman
on 05 Jun 2012

Honestly, in the long run if they do manage to bring up the efficiency, everything from cell phones to outdoor advertisements and sky scrapers would be able to take advantage of solar energy. What do you think?

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